For what it's worth, I don't think it is necessary to install anything. Or to have any desktop at all, lightweight or otherwise.

I'm assuming Steam is a game that requires graphics and hence x. You don't have to have a desktop to start x. You'll have to look a little to find the equivalent in whatever 'buntu you are using but in Lubuntu 13.04 to have an x session with no desktop you delete the line beginning "@pcmanfm . . ." from the file /etc/xdg/lxsession/Lubuntu/autostart. This is what I do all the time now.

A desktop always seemed pointless to me. You could keep a copy of the regular version of the file with the suffix -regular added to the filename and a copy of the edited version with the suffix -steam. Then just copy whichever and rename it to remove the suffix. So find your autostart file and delete the line referring to your file manager.

If you also don't want your panels just start a terminal or use alt-f2 and killall whatever-the-name-of-your-panel-is. Then use alt-f2 to start your game. If you have the disk space it might be more convenient to do this on a seperate installation on another partition. Then you wouldn't need to be renaming config files between the regular version and the steam-only version and you wouldn't need to even have panels or anything else extraneous installed. And of course no un-needed services. On the other hand if you wanted to use the same partition it probably wouldn't be too hard to write a script to rename config files and reboot in order to switch between the regular and the steam-only modes.